Linear indicator devices which comprise a flexible indicator strip or tape displaceable relative to reference indicia in a direction perpendicular to the general viewing direction, have the wellknown advantages of good readability, reduced parallax and small frontal space requirement, which make such devices highly suitable for control panels and instrument boards, grouping numerous indicating devices. Their frontal dimensions are especially small where the indicator strip bears the graduated scale which passes in front of a fixed pointer, a small fraction of the scale being frontally viewable, whereas an indicator strip, with a color break defining the pointer, displaceable along a fixed graduated scale provides a peripherally viewable display without requiring the viewer's concentration, on the same.
In classic linear tape indicating devices, the flexible indicator strip or tape is tensioned in a loop and guided by idler rollers or pulleys, a straight portion of the loop between two successive rollers being visible from the front. The loop is generally endless and the tension is produced by resilient means which either are disposed between ends of the strip or bear excentrically against tensioning pulley. Due to the inertia of the guiding pulleys or rollers and the frictional resistance of their pivots, accurate displacement of the flexible strip or tape is not generally obtained directly from the available energy for displacing a signal but necessitates servo control.
Servo control conventionally involves a servo motor for displacing the strip, a position sensor for the strip, comparator means receiving the input signal representing the value to be displayed and an output signal from the position sensor, the comparator means supplying an error or actuating signal to a servo amplifier for the servo motor. Derivative or integrating components are frequently added to the error signal in order to stabilize the servomechanism and improve its response speed.
Mechanical complications and relatively high power requirements are drawbacks to sought-after compactness in linear display tape indicating devices and lead to costs which are often totally out of proportion with their actual advantages.
Indeed, in the U.S. Pat. No. 3,656,451 to Gear an indicator is disclosed having a self-supporting spring stainless steel tape which is wound on a drum passed over an idler pulley and guided in a channel over only part of its length. This arrangement therefore requires two rotatable parts for mounting and guiding the steel tape. But further, owing to the inherent resiliency of the steel tape, it is necessarily spring biased, against its various guiding surfaces thereby increasing frictional losses and increasing power requirements of such an indicator.